In any romance, there are pivotal firsts -- first kiss, first fight, first vacation as a couple. And if you make it back from that vacation smiling, there could be the make-it-or-break-it M-word: moving in.

Shacking up. Cohabitating. Taking the half-way plunge. It's not a step to be taken lightly, or for the purpose of halving your rent.

Moving in together is the closest you'll get to marriage without the "I do." These days, many people use it as something of a "test run" before deciding to commit for the long haul, and it can offer a nice glimpse into your future as a couple. According to the Huffington Post, a 2007 Rutgers study found that more than 50 percent of U.S. first marriages follow a period of living together. The problem is, if you do it too soon or for the wrong reasons, moving in together can end your relationship prematurely. According to the same article, a different study by the CDC found that of the couples who move in together, almost half break up within five years.

So what's the right time? What are the right reasons?

Every person and every couple is unique, and the circumstances that lead to a 50-year relationship in one case can be an absolute disaster in another. There are, however, some fairly straightforward criteria that can predict whether sharing a space will be the beginning or the end of a beautiful thing.

In this article, we'll find out which questions you should ask yourself and your partner before taking the half-way plunge, throw out some considerations you may have overlooked, and check out some tips that could increase the odds of success.

The first sign of whether or not you're ready to share a home is deceptively simple: Do you know each other?

You've got the ring and planned your proposal, but if you're girlfriend and her family are the traditional types, don't forget the next step. What should you know before you ask her father for her hand in marriage?

So, you've been seeing each other for a few months now and you think he might be Mr. Right (or at least Mr. Long-Term). Does that mean it's time to introduce him to your parents? How and when should you do it?